1. Field of the Invention
The present invention generally relates to an ink jet printer capable of regulating a back pressure of an ink acting on a nozzle at a substantially constant value, and also to a buffer tank for such a regulation of the back pressure.
2. Discussion of Related Art
In the ink jet printer for performing a printing by ejecting an ink from a nozzle onto a recording medium, a variation in a back pressure of the ink acting on the nozzle, which may be caused by a variation in an amount of the ink in an ink cartridge, adversely affects an accuracy in the ink ejection from the nozzle. Hence, it is typical that an ink jet printer is equipped with a mechanism for suppressing the variation in the back pressure of the ink.
An example of such an ink jet printer (as disclosed in U.S. Pat. No. 6,702,427, for instance) is equipped with a buffer tank in which is held an ink whose surface is maintained substantially at a constant level, to suppress the variation in the back pressure acting on the nozzle. This buffer tank is shown in FIGS. 7 and 8 and denoted by reference numeral 100. The buffer tank 100 has a top plate 104, and an ink inlet needle or hollow needle 102 for drawing an ink I in an ink cartridge 101 into the buffer tank 100, and an air outlet needle or hollow needle 103 for flowing the air out into the ink cartridge 101; the ink inlet and air outlet hollow needles 102, 103 are disposed to extend through the top plate 104. In the buffer tank 100, the ink inlet hollow needle 102 extends downward farther than the air outlet hollow needle 103, near a bottom of the buffer tank 100. In this regard, since each of the hollow needles 102, 103 extends through one of two plug members 106 provided to the ink cartridge 101 when the ink cartridge 101 is attached to the buffer tank 100, each of the hollow needles 102, 103 is formed in a hollow needle-like shape.
In a state where the ink I is not ejected from the nozzle, as shown in FIG. 7, the ink I does not flow out of the buffer tank 100 toward the nozzle, and the ink level is held still at a level close to the lower end of the air outlet hollow needle 103 with the ink I present inside a lower end part of the air outlet hollow needle 103. That is, in this state, because the air is not allowed to flow out into the ink cartridge 101 via the air outlet hollow needle 103, the ink I in the ink cartridge 101 is not drawn into the buffer tank 100. When the ink I is ejected from the nozzle in this state, the ink I flows out of the buffer tank through an ink outlet 105 formed in a bottom portion of the buffer tank 100 toward the nozzle, with the ink level of the buffer tank 100 lowered.
Then, the ink surface is separated from the lower end of the air outlet hollow needle 103, with a meniscus being formed around or inside the lower end of the air outlet hollow needle 103. When the ink surface is further lowered and a head difference reaches a certain value ho, the formed meniscus is broken, with the air in the buffer tank 100 flowing out into the ink cartridge 101 through the air outlet hollow needle 103. Then, the ink I in the ink cartridge 101 is drawn into the buffer tank 100 through the ink inlet hollow needle 102 in place of the air. The ink level of the buffer tank 100 accordingly arises and eventually virtually reaches the lower end of the air outlet hollow needle 103. The state where the air in the buffer tank 100 is not allowed to flow out into the ink cartridge 101 is again established, with the flow of the ink I into the buffer tank 100 stopped.
In the buffer tank shown in FIGS. 7 and 8, however, a meniscus is formed around or inside the air outlet hollow needle, when the ink level is lowered. When the ink level has been lowered to the level to make the head difference h0 (a distance between the lower end of the air outlet hollow needle and the ink surface), the meniscus is broken at last, allowing the ink to flow into the buffer tank. That is, the ink level of the buffer tank varies by the head difference ho. Since the air outlet hollow needle is formed of a narrow, hollow needle-like member, it is inevitable that the head difference h0 between the lower end of the air outlet hollow needle and the ink surface which is necessary to break the meniscus is relatively large. Accordingly, the variation in the ink level of the buffer tank is relatively large, making it difficult to hold the back pressure of the ink acting on the nozzle substantially constant. Further, depending on a head difference h0 necessary to break a meniscus, there may be a case where the meniscus formed at the outer periphery of the air outlet hollow needle can not be broken. In this case, the ink supplied to the nozzle may include undesirable air bubbles.
In addition, in the buffer tank as shown in FIGS. 7 and 8, the ink in the ink cartridge flows out or is drawn into the buffer tank via the ink inlet hollow needle which is narrow and extends down to a level near the bottom of the buffer tank. Therefore, the resistance to the flow of the ink flowing into the buffer tank is relatively large. Thus, when the ink level of the buffer tank is lowered upon ejection of the ink from the nozzle which involves the supply of the ink out of the buffer tank, the replenishing the buffer tank with the ink is impeded.
Particularly where the temperature is low or where the viscosity of the ink is relatively high due to evaporation of water in the ink or others, the resistance to the ink flow is further increased, slowing the replenishing the buffer tank with the ink. When the ink level is lowered in such a case, the ink level can not be immediately restored to the original level (close to the lower end of the air outlet hollow needle), leading to deterioration in the constancy of the ink level which in turn adversely affects the constancy of the back pressure of the ink acting on the nozzle. Further, there may be a case where the ink is not drawn into the buffer tank even when the ink level has been lowered near the bottom of the buffer tank; in this case, the ink supplied to the nozzle may include air bubbles.